Polarimetric studies will provide a new probe of cosmic x-ray sources, supplying important clues to source geometries and emission mechanisms. However, at the present time there is only one measurement of x-ray
polarization from a cosmic source, the OSO-8 detection of 19% linear polarization from the Crab Nebula.

We propose a new low cost x-ray polarimeter experiment (XPE), ideally sized for small satellites such as NASA's Small Explorer. With peak sensitivity near 15 keV, the XPE complements the Stellar X-Ray Polarimeter (SXRP, with peak sensitvity near 8 keV) on the international
Spectrum-X-Gamma mission. The x-ray polarimetric sensitivity of XPE is enhanced by a novel geometry for the beryllium scattering block, and by the use of a low-background position-sensitive proportional counter. The entire experiment is characterized by simplicity and reliability. With its higher energy response, XPE probes deep into the observed x-ray sources. Because it is dedicated solely to x-ray polarimetry, XPE will provide observations of a larger number of bright cosmic x-ray sources. XPE will respond to targets of opportunity
such as the bursting pulsar and the galactic superluminal jet x-ray sources. The target list will include pulsing and non-pulsing accreting bright x-ray binaries, an x-ray bright radio pulsar (the Crab), and the AGN Cen A.
Pulsing sources in particular, with their tera-gauss
magnetic fields, should produce strongly polarized x-ray emission. With a 10-20 keV sensitivity of 1% for a 100,000 s observation of Her X-1, and 0.5% for a 25,000 s observation of GRS1915+105 in its high state, XPE
will permit pulse phase spectro-polarimetric studies of pulsing sources, thus constraining the x-ray beam and magnetic field geometry, and probe the stucture and dynamics of cosmic jets. We describe details of
the XPE design, and present a sample observing plan with expected sensitivities.